index+unique constraint 和 unique index 的区别好像执行计划不同~

SQL> drop table test;

Table dropped

SQL> create table test as select * from all_objects;

Table created

SQL> create index ind_TEST on test(object_id);

Index created

SQL> alter table test add constraint un_test unique(object_id) using index ind_test;

Table altered

SQL> insert into test select * from all_objects;

insert into test select * from all_objects

ORA-00001: 违反唯一约束条件 (SCOTT.UN_TEST)

SQL> explain plan for

2  select object_id from test where object_id=32;

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Plan hash value: 2388774261

-----------------------------------------------------------------------------

| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

-----------------------------------------------------------------------------

|   0 | SELECT STATEMENT |          |     1 |    13 |     1   (0)| 00:00:01 |

|*  1 |  INDEX RANGE SCAN| IND_TEST |     1 |    13 |     1   (0)| 00:00:01 |

-----------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("OBJECT_ID"=32)

13 rows selected

SQL> alter table TEST

2    drop constraint UN_TEST cascade;

Table altered

SQL> drop index ind_test;

Index dropped

SQL> create unique index un_ind_test on test(object_id);

Index created

SQL> insert into test select * from all_objects;

insert into test select * from all_objects

ORA-00001: 违反唯一约束条件 (SCOTT.UN_IND_TEST)

SQL> explain plan for

2  select object_id from test where object_id=32;

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Plan hash value: 3033676969

--------------------------------------------------------------------------------

| Id  | Operation         | Name        | Rows  | Bytes | Cost (%CPU)| Time

--------------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |             |     1 |    13 |     1   (0)| 00:00:01

|*  1 |  INDEX UNIQUE SCAN| UN_IND_TEST |     1 |    13 |     1   (0)| 00:00:01

--------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("OBJECT_ID"=32)